The demo for the upcoming Xbox 360 RTS exclusive, Halo Wars is available now on the Marketplace. As also mentioned by Major Nelson. So, if you haven't already I would suggest downloading it now before god only knows what happens to those servers. 

The wait to experience the genre-defining strategy game “Halo Wars” ends today with the release of the “Halo Wars” demo on Xbox LIVE*. The demo introduces players to the game’s groundbreaking control scheme through two tutorials, gives a taste of the epic story with the first two campaign missions, and allows people to experience the intense, competitive action offered via one of the game’s 14 Skirmish maps.

In addition to the launch of the “Halo Wars” demo on Xbox LIVE today, Microsoft Game Studios invites fans to experience the action and take a closer look at the strategies and decisions that lie ahead, in the riveting new “Call to Battle: ‘Halo Wars’ Trailer.”

Featuring exciting new battle sequences between the UNSC and Covenant, weaved in with scenes from the beautiful in-game cinematics, the trailer showcases what fans can expect when they take command in “Halo Wars,” the action strategy game developed exclusively for Xbox 360. This new trailer is available for download on Xbox LIVE and Xbox.com/Halo.

Check out a few screenshots and trailer below...